Semiconductor laser arrays have recently attained optical power outputs which make them attractive for many high power applications, such as for pumping solid state lasers. For example, GaAlAs buried heterostructure lasers produced with silicon impurity induced disordering (IID) have achieved low threshold, high efficiencies, and power levels of up to 0.5 watt. One limitation, however, on output power capability for GaAlAs lasers is catastrophic damage to the laser facets or mirrors, due in part to local heating at high output powers from optical absorption in the active regions near the facets. Additionally, at somewhat lower power levels, facet erosion caused by oxidation of the active region may occur thereby reducing the useful life of the laser. It is known that improved catastrophic damage levels can be obtained either by shifting the laser emission to a longer wavelength, i.e. lower energy, relative to the threshold absorption energy at the facet or by increasing the bandgap of the facet material, i.e. the absorption energy at the facet, relative to the laser emission energy. In either instance, regions adjacent to the facets characterized by substantially lower optical absorption are created. These regions are called "windows" and lasers having these facet windows are called "window lasers".
Window lasers have been fabricated by a variety of methods. One method is to introduced a change of material composition by the selective diffusion of zinc into the laser cavity everywhere except at the facets while maintaining a constant thickness waveguide layer. The diffuxed zinc shifts the laser emission to a longer wavelength. The power output of zinc diffuxed window lasers is limited by catastrophic damage due to local heating in the bulk rather than at the facets.
More recently various methods have been used to increase the effective bandgap at the facet, including zinc impurity induced disordering (IID) and etching and regrowth. Blauvelt et al in Applied Physics Letters, Vol. 40(12), pp. 1029-1031, June 15, 1982, describe a buried heterostructure window laser produced by the latter method. Portions of the active layer which will eventually form the window regions are removed by selective etching, then a thicker transparent waveguide or window or wider bandgap material is regrown in its place. Yonezu et al in IEEE Journal of Quantum Electronics, Vol. QE-15, pp. 775-781, 1979, discloses the use of the Zn diffusion technique in forming a window laser. More recently, Y. Suzuki et al, "Fabrication of GAAlAs `Window Stripe` Multiple-Quantum Well Heterostructure Lasers Utilising Zn Diffusion-Induced Alloying", Electronic Letters, Vol. 20(9), pp. 383-384, Apr. 26, 1984, disclosed the formation of Zn impurity diffusion window regions using Si.sub.3 N.sub.4 masking in fabricating a single emitter window laser having a multiple quantum well active region.
Another method for increasing the available power level limited by catastrophic facet damage is to grow very thin active layers to lower the optical power density at the facets. In order to avoid increasing the threshold current because of a decrease of gain, nonuniformity of the active layer (for liquid phase epitaxy grown layers), and the influence of the active layer interfaces, the active layer is tapered, i.e. made thinner only near the facets. Burnham et al in U.S. Pat. No. 4,546,480 and Murumaki et al in Electronics Letters, Vol. 22(4), pp. 217-218, Feb. 13, 1986, disclose two distinct lasers produced by this technique.
It is desirable, when fabricating window regions in semiconductor lasers, not to significantly increase scattering, propagation and diffraction losses particularly at the transition between the window region and the active region. In order to reduce diffraction loss in the vertical direction perpendicular to the plane of the active region, it is preferred that a waveguide remain in the window regions to confine the light. Window formation techniques, including IID when taken to an extreme extent, may completely destroy any waveguiding capabilities present in the as-grown layers, in particular the vertical waveguiding properties in the window region, since the entire region after such treatment will completely disordered when taken to extreme extent. For example, see the representations concerning this effect in U.S. Pat. Nos. 4,511,408 and 4,639,275 wherein complete disordering treatment is observed. Other window formation techniques, such as etching and regrowth, can introduce abrupt boundaries and transitions that cause large scattering losses. Thinning the active region can cause propagation losses which depend on the degree of thinning and the length of the window regions.
The technique of IID has been developed as a means for crafting semiconductor structures. This technique may be defined as a process of enhanced rate of interdiffusion of ordered elemental constituents as initially formed in consecutively deposited layers of semiconductor compounds or alloys through the introduction, i.e., diffusion of an impurity into the layers. The utility of IID, as discussed in K. Meehan et al, "Disorder of an Al.sub.x Ga.sub.1-x As-GaAs Superlattice by Donor Diffusion", Applied Physics Letters, Vol. 45(5) pp. 649-651, Sept. 1, 1984 and in U.S. Pat. No. 4,639,275, is useful in fabricating planar index waveguide structures in multilayer GaAs/GaAlAs semiconductor structures, such as for integrated optical waveguides and heterostructure lasers and array lasers. An example of its application in the fabrication of buried heterostructure lasers is disclosed in the article of R. L. Thornton et al entitled, "Highly Efficient, Long Lived AlGaAs Lasers Fabricated by Silicon Impurity Induced Disordering", Applied Physics Letters, Vol 49(3), pp. 133-134, July 21, 1986.
It has been recognized that impurity diffusion into III-V compound semiconductors is an important step in the fabrication of optoelectronic devices. Recently, much attention has been given to the diffusion of Si into GaAs. Also, recently, considerable attention has been given to impurity induced disordering (IID) in GaAs/GaAlAs quantum well structures.
In particular, the diffusion of silicon in GaAs has been uder study and investigation for many years. As an example, G. R. Antell in an article, "The Diffusion of Silicon in Gallium Arsenide", Solid-State Electronics, Vol. 8, pp. 943-946 (1965), discloses the diffusion of Si into GaAs carried out at high temperatures in a sealed quartz capsule containing an overpressure of As to prevent the outdiffusion of As from the GaAs. The diffusivity and activation of Si in GaAs is proportional to the As overpressure and the Ga vacancy concentrations. Excess As pressure in a closed ampoule is required for successful diffusion. See, also, the more recent article on this subject of E. Omura et al, "Closed-Tube Diffusion of Silicon in GaAs From Sputtered Silicon Film", Electronic Letters, Vol. 22(9), pp. 496-498 (Apr. 24, 1986).
More recently, the effects of encapsulation relative to Si implanted into GaAs have been studied to prevent the outdiffusion from GaAs and provide, in some cases, a source of Si for diffusion into GaAs. See the article of T. Onuma et al, "Study of Encapsulants for Annealing Si-Implanted GaAs", Journal of Electrochemical Society, Vol. 129(4), pp. 837-840 (April, 1982). Diffusion of Si was enhanced by SiO.sub.2 encapsulation but was negligible with Si.sub.2 N.sub.4 encapsulation or when capless. The activation of the diffusion process is initiated at high anneal temperatures, such as 750.degree. C. and above. Onuma et al indicates that the SiO.sub.2 cap layer is permeable to Ga while the Si.sub.3 N.sub.4 cap layer is impermeable to Ga and As so that SiO.sub.2 permits the diffusion of Ga to provide for Ga vacancies in the GaAs and the substitution of Si. However, due to the deposition method employed, cracks developed in some of the samples when subjected to the subsequent high temperature annealing process.
M. E. Greiner et al in the article, "Diffusion of Silicon in Gallium Arsenide Using Rapid Thermal Processing: Experiment And Model", Applied Physics Letters, Vol. 44(8), pp. 750-752 (Apr. 15, 1984) examines Si diffusion from a thin elemental deposited source thereof using rapid thermal processing with several different encapsulants. The results show that diffusion was dependent on the type of encapsulant. The Si source layer and the encapsulants were deposited at relatively low temperatures, i.e. below 450.degree. C., with subsequent annealing being accomplished at high temperatures of 850.degree. C.-1050.degree. C. High concentrations of Si diffused into GaAs resulted from a SiO.sub.2 capped thin Si source layer. In particular, a model proposed by Greiner et al explains that paired Si atoms can move substitutionally by exchaning sites with either Ga or As vacancies which explains the enhanced diffusion when using an SiO.sub.2 cap.
More recently, K. L. Kavanagh et al in the articles, "Silicon Diffusion at Polycrystalline-Si/GaAs Interfaces", Applied Physics Letters, Vol. 47(11), pp. 1208-1210 (Dec. 1, 1985) and "The Polycrystalline-Si Contact to GaAs", Journal of the Electrochemical Society, Vol. 133(6), pp. 1176-1179 (June, 1986), reveals that, under proper conditions, the addition of As to the Si source layer revealed further enhanced diffusion, i.e. greater concentrations of Si into the GaAs. These conditions called for depositing hydrogenated amorphous Si (a-Si:H) onto GaAs in a silane plasma at 450.degree. C. and subsequent annealing at temperatures between 600.degree. C.-1020.degree. C. The results showed that high level interdiffusion of Si atomic pairs with Ga and As vacancies occurs when As is initially added to the Si source layer. However, the surface area of films deposited onto GaAs continued to have a large number of randomly spaced bubbles, indicative of compressive stresses in the film, developed after the high temperature annealing process.
In copending patent applications Ser. Nos. 117,593 and 117,596, filed on Nov. 5, 1987, assigned to the same assignee herein and incorporated herein by reference thereto, it has been recognized that complete disordering in selected regions of deposited layers of semiconductor compounds or alloys can be prevented by controlled IID so that partial disordering will occur on a reproducible basis forming integrated transparent waveguide sections in a monolithic opto-electronic device or chip. In application Ser. No. 117,593, a thin film bilayer composite source is deposited on a semiconductor structure for incorporating an impurity species or dffusant, such as Si, with the level of impurity incorporation and the depth of impurity penetration into the crystal bulk being reproducibly controlled by IID processing employing a thermal anneal treatment. In application Ser. No. 117,596, a thin film bilayer composite source is deposited on a semiconductor structure for incorporating an impurity species or dffusant, such as Si, with the level of impurity incorporation and the depth of impurity penetration into the crystal bulk being reproducibly controlled by IID processing employing a rapid thermal anneal (RTA) treatment followed by a slow thermal anneal (STA) treatment. An object of the present invention is to produce a semiconductor window laser using IID without substantial scattering, propagation and diffraction losses in the laser cavity and, in particular, rendering the window region of the laser nonabsorbing to radiation wavelength of the active region while incorporating a refractive index waveguiding mechanism in the window regions of such lasers.